Specific patterns of neuronal firing induce changes in synaptic strength that may contribute to learning and memory. If the postsynaptic NMDA (N-methyl-D-aspartate) receptors are blocked, long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission and the learning of spatial information are prevented. The NMDA receptor can bind a protein known as postsynaptic density-95 (PSD-95), which may regulate the localization of and/or signalling by the receptor. In mutant mice lacking PSD-95, the frequency function of NMDA-dependent LTP and LTD is shifted to produce strikingly enhanced LTP at different frequencies of synaptic stimulation. In keeping with neural-network models that incorporate bidirectional learning rules, this frequency shift is accompanied by severely impaired spatial learning. Synaptic NMDA-receptor currents, subunit expression, localization and synaptic morphology are all unaffected in the mutant mice. PSD-95 thus appears to be important in coupling the NMDA receptor to pathways that control bidirectional synaptic plasticity and learning.

No abstract

Zinc is present in presynaptic nerve terminals throughout the mammalian central nervous system and likely serves as an endogenous signaling substance. However, excessive exposure to extracellular zinc can damage central neurons. After transient forebrain ischemia in rats, chelatable zinc accumulated specifically in degenerating neurons in the hippocampal hilus and CA1, as well as in the cerebral cortex, thalamus, striatum, and amygdala. This accumulation preceded neurodegeneration, which could be prevented by the intraventricular injection of a zinc chelating agent. The toxic influx of zinc may be a key mechanism underlying selective neuronal death after transient global ischemic insults.

Background and Purpose: Infarct volume is one of the common indexes for assessing the extent of ischemic brain injury following focal cerebral ischemia. Accuracy in the measurement of infarct volume is compounded by postischemic brain edema that may increase brain volume in the infarcted region. We evaluated the effect of brain edema on infarct volume determined by triphenyltetrazolium chloride and hematoxylin and eosin stains in a focal cerebral ischemia model in rats.Methods: In a middle cerebral artery occlusion model in rats, infarction is confined to the cerebral cortex. The infarct was delineated by triphenyltetrazolium chloride stain and, in selected samples, by hematoxylin and eosin stain. We determined infarct size at different times after the ischemic insult (6 hours to 7 days) in relation to the evolution of brain edema by the direct measurement of infarct volume. Indirect measurement to reduce the effect of edema on infarct volume was also conducted in the same brain samples.Results: Direct measurement showed that infarct volume fluctuated with the evolution of brain edema (one-way analysis of variance, p

This study was designed to determine whether formalin fixation alters diffusion parameters in the infarcted brain. Diffusion tensor images were obtained from anesthetized mice 1 hr after middle cerebral artery occlusion and repeated after formalin fixation of brains. In live animals, there was a significant decrease in the trace of the diffusion tensor (Tr(D)) in infarcted cortex and external capsule compared with contralateral brain areas, with no change in relative anisotropy (RA Diffusion tensor imaging (DTI) is a widely used tool for the noninvasive investigation of tissue pathology and morphology. DTI examination of fixed tissue has the advantage that imaging times can be very long. As a result, it is possible to obtain data with high signal-to-noise ratio (SNR) at extremely high spatial resolution compared with in vivo studies (1). Previously, we demonstrated that the anisotropy of water diffusion in formalin-fixed normal mouse brain is the same as that observed in vivo (2). However, a significant decrease in the trace of the diffusion tensor (Tr(D)) after fixation was also observed. Close examination of the ratio of Tr(D) between in vivo and ex vivo measurements (R in/ex ) suggests that the change in Tr(D) is not uniform across the entire brain. For example, R in/ex (mean Ϯ SD, n ϭ 7) is 2.0 Ϯ 0.3, 3.0 Ϯ 0.3, and 3.5 Ϯ 0.3 in cortex, anterior commissure, and corpus callosum, respectively (2). Differential regional changes in diffusion magnitude after fixation result in an altered image contrast of ex vivo Tr(D) maps. Thus, the sensitivity of in vivo diffusion MRI to brain injury may be affected after fixation. The question also remains of whether water diffusion anisotropy remains unchanged in injured brain after fixation. The purpose of this study is to address both questions using a mouse model of focal cerebral ischemia. METHODS DTI of Mouse BrainsSix male mice (C57BL/6) weighing 20 -30 g were subjected to permanent middle cerebral artery occlusion (MCAO) as detailed elsewhere (3). In vivo DTI was conducted on each mouse 1 hr after MCAO as described previously (2). At the conclusion of the in vivo examination, mice were perfusion-fixed, under anesthesia, through the left cardiac ventricle with phosphate-buffered saline (PBS) followed by 10% formalin in PBS (4). The intact brain was excised, placed in 10% formalin/PBS, and stored at 4°C for 2 weeks before imaging. Fixed brains were placed in a 1-cm inner diameter solenoid coil to serve as the RF transmitter and receiver. Ex vivo DTI acquisition was performed following the exact protocol as previously reported (2). The spatial resolution of the ex vivo images was set equivalent to that of the in vivo images to facilitate comparison between the two data sets (117 ϫ 117 ϫ 500 m 3 ). Similarly, the diffusion sensitizing gradients were set so the degree of signal attenuation due to diffusion was comparable for in vivo and ex vivo measurements. This necessitated using a larger b value for the ex vivo studies (1.813 versus 0.768 ms/m 2 ) due to the significant...

Robust expression of TSP-1 and TSP-2, 2 major angiostatic factors, was noted in the ischemic brain with different temporal expression profiles from different cellular origins. The expression of these angiostatic factors, especially TSP-2, likely contributes to the spontaneous resolution of postischemic angiogenesis. Further studies are needed to explore the molecular mechanisms that regulate the balance of angiogenic and angiostatic factors in the ischemic brain.

Background and Purpose-Genetically engineered mice are used to study the role of single genes in cerebral ischemia, but inherent, strain-dependent differences in neuronal vulnerability may affect experimental end points. To examine this possibility, tissue injury resulting from focal ischemia and its relationship to cerebral hemodynamics were determined in 3 common mutant mouse strains. Methods-Permanent middle cerebral artery ligation was performed in male C57BL/6J, Balb/C, and 129X1/SvJ mice.Mean arterial blood pressure, blood gases, basal and postischemic cortical blood flow ([ 14 C]iodoantipyrine autoradiography and laser-Doppler flowmetry), posterior communicating artery patency, and infarct size were determined. Results-Basal cortical blood flow did not differ among strains. Ten minutes after middle cerebral artery ligation, relative red cell flow in the ischemic cortex was 6% to 7% of preischemic flow in every strain. Despite similar hemodynamics, cortical infarcts in Balb/C mice were 3-fold larger than those in 129X1/SvJ and C57BL/6J mice; infarct size in the latter 2 strains was not significantly different. The posterior communicating artery was either poorly developed or absent in Ͼ90% of the Balb/C and C57BL/6J but in Ͻ50% of the 129X1/SvJ mice. Conclusions-The extent of ischemic injury differed markedly between the 3 strains. The presence and patency of posterior communicating arteries, although variable among strains, did not affect preischemic or postischemic cortical blood flow or bear any relationship to ischemic injury. Therefore, intrinsic factors, other than hemodynamic variability, may contribute to the differences in ischemic vulnerability among strains. These findings underscore the importance of selecting genetically matched wild-type controls. (Stroke. 2000;31:2707-2714.)

Pleiotrophin (PTN) is a heparin-binding, 18 kDa secretory protein that functions to induce mitogenesis, angiogenesis, differentiation, and transformation in vitro. PTN gene (Ptn) expression is highly regulated during development and is highest at sites in which mitogenesis, angiogenesis, and differentiation are active. In striking contrast, with the exception of the neuron, the Ptn gene is only minimally expressed in adults. We now demonstrate that Ptn gene expression is strikingly upregulated within 3 d in OX42-positive macrophages, astrocytes, and endothelial cells in areas of developing neovasculature after focal cerebral ischemia in adult rat. Ptn gene expression remains upregulated in these same cells and sites 7 and 14 d after ischemic injury. However, expression of the Ptn gene is significantly decreased in cortical neurons 6 and 24 hr after injury and is undetectable in degenerating neurons at day 3. Neurons in contralateral cortex continue to express Ptn in levels equal to control, uninjured brain. It is suggested that PTN may have a vital role in neovascular formation in postischemic brain and that postischemic brain is an important model in which to analyze sequential gene expression in developing neovasculature. In contrast, Ptn gene expression in injured neurons destined not to recover is strikingly reduced, and potentially its absence may contribute to the failure of the neuron to survive.